In response to environmental concerns and legislative action, manufacturers of electronic devices are switching from lead-containing (Pb-containing) to lead-free (Pb-free) solders. The transition from Pb-containing to Pb-free solders has not been without problems. The most widely used Pb-free solders in the electronics industry contain a high tin (Sn) content, typically in excess of 94 wt. %, and further contain silver (Ag), copper (Cu), or both, and possibly other elements such as nickel (Ni), cobalt (Co), zinc (Zn), bismuth (Bi), etc. Solder alloys that contain Sn, Ag, and Cu are referred to as SAC solders. If they contain an additional element they are usually referred to as SACX, where X represents the additional element. SAC and SACX Pb-free solders typically have a higher melting point than eutectic PnSn solders. Therefore, the required peak reflow temperatures are higher for Pb-free solders than for eutectic Pb/Sn solders. These higher reflow temperatures can lead to undesirable thermal loading effects including those associated with differential thermal expansion-induced stresses. SAC and SACX Pb-free solders have a higher elastic modulus and yield point than PbSn solders (see, for example, the NIST web site). Furthermore, the yield point of Pb-free alloys, which essentially puts a limitation on the magnitude of the stress the joint can be exposed to, is more sensitive to the strain rate of the applied stress than for PbSn alloys. These combined mechanical properties of Pb-free solders tend to make joints that are more susceptible to brittle failure than joints made from PbSn solders. This is especially true when the joints are exposed to stresses applied at high strain rates, such as those that may occur during testing, handling, and assembly. Packages that contain solder spheres, typically referred to as solder balls or solder bumps, as part of the overall device electrical interconnect are prone to high strain rate brittle fracture. One such problem is referred to as solder ball drops, or missing solder spheres.
FIG. 1 shows the bottom side of a ball grid array (BGA) package 1 that will eventually be attached to another electrical circuit, such as a circuit board. The electrical and mechanical interconnect of the package 1 to a circuit board may be made via solder spheres 20, typically referred to as solder balls 20. It will be appreciated that here and in the rest of this disclosure none of the figures are to scale; the sizes of some items have been exaggerated, while others have been reduced, for a presentation that is easier to see. A BGA 5 of the package 1 comprises a substrate 10, which is typically a laminate made from conductive and non-conductive layers. The conductive layers connect an integrated circuit embedded in polymer 7 on a front side of the substrate 10 to the bottom side of the substrate 10. The bottom side of the substrate 10 has exposed metal pads 11 onto which solder balls 20 can be attached. The exposed metal pads 11 are typically comprised of Cu, which provides the conductive layers in the substrate 10, coated with an electrolytic deposit of a bi-layer nickel/gold (Ni/Au) film. The solder balls 20 may be used to subsequently solder the electrical device 1 onto a circuit board or other electrical component.
FIG. 2 is a side-view of a portion of the electrical device 1 just prior to solder reflow of the balls 20 to the BGA pads 11. The substrate 10 includes a non-conductive core layer 12, which is typically bismaleimide triazine (BT). Cu traces 14 are disposed on both sides of the core layer 12 in complex three dimensional patterns with an array of via through holes such that the Cu traces 14 provide an electrical connection from the die interconnect to the bottom side of the BGA substrate 10. Atop the Cu traces 14 is a polymer film (not shown in the figure), typically referred to as a solder mask. The polymer film has holes that selectively expose the Cu trace 14. The exposed regions in the Cu layer 14 are subsequently coated with a bi-layer Ni/Au film 17 via electrolytic plating. The gold (Au) layer 18 provides a wettable surface for solder ball 20 attachment. Because Cu is a fast diffuser in Au, a barrier that prevents fast migration of Cu through the Au layer 18 is often used to insure that the Au surface maintains it's wettability to solder 20. This barrier is provided by the Ni layer 16. In addition, the Ni layer 16 is both metallurgically compatible with the solder 20 and prevents migration of the Cu from the Cu layer 12 into the solder 20. The Au layer 18 further protects the Ni layer 16 from oxidation. Typical metallic layer thicknesses are 10 to 70 μm for the Cu layer 14, 2 to 10 μm for the Ni layer 16, and 0.05 to 2 μm for the Au layer 18.
FIG. 3 is a side-view showing a solder ball 20 bonded to its respective pad 11. Although not shown, flux is generally disposed over the Au layer 18 or onto the solder balls 20 prior to solder ball attachment. The flux is used both to remove the naturally occurring metallic oxides that are present on the solder balls 20, as well as to hold the solder balls 20 in place until the solder reaches it's melting point, at which time the solder 20 reacts with the pad 11 metallization. After fluxing, the solder balls 20 are placed on top of the BGA pads 11. After solder ball 20 placement, the substrate 10 is then inserted into a reflow oven, during which time the solder 20 is melted and wets the BGA pads 11. The Pb-free solder balls 20 are typically an alloy of tin (Sn), silver (Ag) and copper (Cu), although other Pb-free alloys can also be used
The reflow process heats and then cools the solder ball 20 to bond the solder ball 20 to the metallization layers of the pad 11. This reflow process can be quite intricate, and a detailed discussion is beyond the scope of this disclosure. The upshot, however, is that when the solder 20 melts, the Au layer 18 in contact with the melted solder 20 goes quite rapidly into solution into the solder 20, thus exposing the Ni layer 16 to the solder 20. The solder 20 reacts with the Ni 16, and forms an intermetallic compound (IMC) region 19 that mechanically binds the solder 20 with the Ni layer 16. The IMC 19 is typically composed of Ni and Sn (NiSn); Ni, Sn and Cu (NiCuSn); or Ni, Cu, Au, and Sn and can be quite thin, from about 0.1 μm to about 5 μm, depending upon the amount of Cu in the SAC, the thermal profile used for reflow, and the number of reflow cycles the device 1 is exposed to.
The IMC 19 may be thought of as the “glue” that holds the solder ball 20 to the substrate 10. Defects in this glue can lead to dropped solder balls, as shown in FIG. 1 with pads 11a. Pads 11a without solder balls 20 cannot be electrically connected to a circuit board and thus are electrical failure points of the device 1. Stress imparted to the solder balls 20 during testing, shipping, assembly, accidental dropping or the like may cause the solder balls 20 to separate from their respective pads 11, and thus lead to electrical failure. Such separation (or ball drop) results from brittle fracture in and around the solder 20 to pad 11 metallization interface; that is, within the IMC region 19.
To address this issue, the prior art has sought to adjust the Pb-free alloy composition. The most widely used composition of SAC is an alloy containing 3 to 4 weight percent (wt. %) of Ag, 0.5 to 1 wt. % of Cu, and 95 to 96.5 wt. % of Sn. Reduction in the Ag content reduces the yield strength of the alloy. Since the maximum stress that the solder joint experiences during mechanical loading is essentially determined by the yield strength of the alloy, a lower Ag content implies less stress on the joint when the solder 20 is exposed to high strain rate mechanical loading. Thus, lowering the Ag content is expected to provide a solder joint that is less prone to brittle failure in the IMC 19 when exposed to high strain rate stresses. Although some researchers have found low Ag content solders are less prone to brittle failure during drop testing, other data is not as clear. This may imply that there are other mechanisms playing a role in the brittle nature of the joint.
Accordingly, there is an immediate need for improved soldering methods, and related electrical devices, that are less prone to brittle failure in solder joints.